The present invention relates to a workpiece holder for holding a workpiece such as a semiconductor wafer, a semiconductor fabricating apparatus having the workpiece holder, a semiconductor inspecting apparatus, a circuit pattern inspecting apparatus, a charged particle beam application apparatus, a calibrating substrate, a workpiece holding method, a circuit pattern inspecting method, and a charged particle beam application method.
With the trend toward finer-geometries of circuit-patterns of semiconductor wafers, a circuit pattern inspecting apparatus employing electron beams has come to be put into practical use.
Techniques concerning such a circuit pattern inspecting apparatus have been described, for example, in Japanese Patent Laid-open No. Sho 59-192943, Japanese Patent Laid-open No. Hei 05-258703, Sandland, et al., xe2x80x9cAn electron-beam inspection system for x-ray mask productionxe2x80x9d, J. Vac. Sci. Tech. B, Vol. 9, No. 6, pp. 3005-3009 (1991), Meisburger, et al., xe2x80x9cRequirements and performance of an electron-beam column designed for x-ray mask inspectionxe2x80x9d, J. Vac. Sci. Tech. B, Vol. 9, No. 6, pp. 3010-3014 (1991), Meisburger, et al., xe2x80x9cLow-voltage electron-optical system for the high-speed inspection of integrated circuitsxe2x80x9d, J. Vac. Sci. Tech. B, Vol. 10, No. 6, pp. 2804-2808 (1992), Hendricks, et al., xe2x80x9cCharacterization of a New Automated Electron-Beam Wafer Inspection Systemxe2x80x9d, SPIE Vol. 2439, pp. 174-183 (Feb. 20-22, 1995).
To inspect a circuit pattern, which comes to be formed in finer-geometries on a wafer having an increased diameter, highly accurately at a high throughput, it is necessary to acquire a pattern image with a higher SN ratio at a very higher speed. To satisfy such a requirement, it is required to keep a higher SN ratio by ensuring the necessary number of electrons to be emitted to the circuit pattern using a large current beam which is equal to or more than 100 times (10 nA or more) that used for a usual scanning electron microscope (SEM), and further, it is essential to highly efficiently detect secondary electrons produced from a substrate and reflection electrons reflected therefrom at a higher speed.
On the other hand, to prevent a semiconductor substrate having an insulating film such as a resist from being affected by electrification, the semiconductor substrate is irradiated with a low acceleration electron beam of 2 KeV or less. The technique is described on pages 622-623 inxe2x80x9cElectron and Ion Beam Handbookxe2x80x9d 2nd Edition, edited by 132nd committee of Japan Society for the Promotion of Science, published by Nikkan Kogyo Simbun, Ltd. (1986). A large current and low acceleration electron beam, however, makes it difficult to observe a circuit pattern at a high resolution because it produces aberration due to the space-charge effect.
A method of solving such a problem has been known in which a high acceleration electron beam is retarded directly before a workpiece to irradiate the workpiece with a substantially low acceleration electron beam. The technique has been described, for example, in Japanese Patent Laid-open Nos. Hei 02-142045 and Hei, 06-139985.
The outline of one example of an electro-optical system of a prior art circuit pattern inspecting apparatus will be described below with reference to FIG. 9. FIG. 9 is a schematic view of an electro-optical system of the prior art circuit pattern inspecting apparatus.
A primary electron beam 201 emitted from an electron gun 1 with a voltage applied to an extraction electrode 2 passes through a condenser lens 3, a scanning deflector 5, an aperture 6, an objective lens 9 and the like to be converged and deflected onto a substrate 10 for a semiconductor device placed on workpiece stages 11 and 12. To retard the primary electron beam 201, a retarding voltage is applied from a high voltage power source 23 to the substrate 10. The irradiation of the substrate 10 with the primary electron beam 201 produces secondary electrons 202 from the substrate 10. The secondary electrons 202 are accelerated to an energy of several KeV by the retarding voltage. An EXB deflector 8 is provided on the electron gun side of the objective lens 9 in such a manner as to be adjacent to the objective lens 9.
The EXB deflector 8 is adapted to cancel the deflection amounts of the primary electron beam 201 due to an electric field and a magnetic field each other and to deflect the secondary electrons 202 by superimposition of the deflection amounts of the secondary electrons 202 due to the electric field and magnetic field. The accelerated secondary electrons 202 thus deflected by the EXB deflector 8 is attracted by an electric field formed by an attraction voltage applied between a detector 13 and an attraction electrode 14 mounted around the detector 13, to enter the detector 13.
The detector 13 is configured as a semiconductor detector. The secondary electrons 202 having entered the semiconductor detector produce electron-positive hole pairs which are then taken out as a current to be converted into an electric signal. The output signal is amplified by a pre-amplifier 21, to become a brilliance modulation input for forming an image signal. After an image of one region of the substrate is acquired by the above operation of the electro-optical system, the image output signal is delayed for a time corresponding to one image plane, and then an image of a second region is similarly acquired. These two images are compared with each other by an image comparing/evaluating circuit, to thus detect a defective portion of the circuit pattern. Here, the irradiated position with the primary electron beam 201 is determined as a position of the substrate on which the electron beam is impinged on the basis of a scanning-deflection signal inputted in the scanning deflector 5.
If the surface height of the substrate is varied by warping of the wafer or the like, however, the irradiated region of the substrate with an electron beam is substantially varied although the electron beam is scanned on the basis of the same deflection signal, that is, the same beam deflection cannot be obtained between two irradiated regions of the substrate.
To solve such a problem, a prior art electron beam application apparatus such as an electron beam plotting apparatus has adopted the following deflection correcting manner:
(1) A sample with standard marks formed on at least two surfaces different in thickness is provided at the outermost peripheral portion of a workpiece stage, and a positional offset between image signals obtained from the standard marks having the different heights is calculated.
(2) The height of each standard mark is converted into a signal by operating an optical sensor for successively measuring the surface height of the workpiece.
(3) A deflection correcting table corresponding to the height is calculated on the basis of the height signal of each standard mark and the positional offset between image signals of the standard marks. The deflection correcting table is stored, and upon observation of the substrate, the deflection is corrected by calculating a deflection correcting signal corresponding to the surface height of the substrate on the basis of the deflection correcting table.
With this technique, even if the surface height of a substrate is varied by warping of the wafer or the like, two regions of the substrate which are different in surface height can be equally irradiated with an electron beam on the basis of a corrected deflection signal.
The technique has been described, for example, in Japanese Patent Laid-open No. Sho 56-103420. According to this technique, the deflection correcting table can be simply updated by repeatedly observing the standard marks provided at the outer peripheral portion of the workpiece stage on which the wafer is left mounted. As a result, even if there occurs a drift of the deflection amount of a primary electron beam due to the change in the electro-optical system with the elapsed time, the deflection can be corrected in such a manner as to keep up with the change with the elapsed time by re-observing the standard marks about several ten times at a specific period of time during processing of one wafer, and updating the deflection correcting table for each re-observation.
A circuit pattern inspecting apparatus adopting the above-described deflection correcting method, however, has been not realized so far.
The gist of the present invention is to provide a circuit pattern inspecting apparatus adopting the above-described deflection correcting method for coping with the warping of a wafer. However, if the circuit pattern inspecting apparatus adopts the above deflection correcting method as it is, there occurs the following problems:
Since a retarding voltage is applied to a substrate, a primary electron beam is affected by an electric field caused by the retarding voltage directly before reaching the substrate.
In general, since a change in electric field is distributed axisymmetrically with respect to the central axis of the primary electron beam, the primary electron beam can be deflected to a desired region by uniformly adjusting the deflection sensitivity irrespective of the position of the wafer. However, at the outer peripheral portion of the wafer, there is produced a nonaxisymmetric disturbance of an electric field caused by the retarding voltage due to the sectional shape of the wafer itself and the sectional structure of an end portion of the workpiece stage on which the water is mounted.
In the circuit pattern inspecting apparatus, since a signal is obtained by one large current scanning only, a retarding voltage being as high as several times or more that used for another electron beam application apparatus is required to restrict the beam diameter into a desired value and to allow a low acceleration electron beam to be impinged on the substrate. Accordingly, the amount of change in electric field caused by the retarding voltage becomes larger than that for another electron beam application apparatus.
As a result, depending on whether or not an observation region of the substrate, that is, an irradiated region with a primary electron beam is located near the outer peripheral portion of the wafer, there occurs an unnegligible difference of the beam, calledxe2x80x9cbeam deflectionxe2x80x9d, between the two irradiated regions located near the outer peripheral portion and in the central portion of the wafer although these regions are irradiated with the primary electron beam on the basis of the same deflection signal.
In such circumstances, even if a deflection correcting table is prepared by providing, like another electron beam application apparatus, standard marks formed on at least two surfaces different in thickness at the outermost peripheral portion of a workpiece stage, the deflection correcting table is affected by the beam distortion inherent to the outer peripheral portion of the workpiece stage. As a result, a suitable deflection correcting signal for the associated position cannot be obtained by referring to the deflection correcting table on the basis of the measurement result of the surface height of the workpiece at the central portion of the workpiece stage, giving rise to a problem in which there occurs a deviation in irradiated position with the electron beam.
The deviation in irradiated position with the electron beam leads to a deviation in pixel in an image signal obtained from the deviated position, thereby causing degradation of the accuracy in comparative inspection of the images. If the deviation in pixel exceeds a specific allowable range, there occurs a problem that the inspection accuracy is critically degraded in the circuit pattern inspecting apparatus aimed at comparative inspection of images.
On the other hand, of a semiconductor fabricating apparatus and a semiconductor inspecting apparatus, an electron beam application apparatus for processing a workpiece by irradiating a workpiece with an electron beam or inspecting the workpiece using an electron beam is required to emit an electron beam in vacuum. Further, to improve the processing accuracy of a workpiece or improving the resolution of an image obtained upon inspection, it is required to control the irradiation energy intensity of the emitted electron beam.
In recent years, the electron beam application apparatus such as an electron beam plotting apparatus for processing a pattern of a semiconductor by irradiating it with an electron beam, a length measuring SEM (scanning electron microscope) for measuring a width or the like of a pattern on the surface of a semiconductor, or an analyzing SEM for analyzing the material of a semiconductor by irradiating the semiconductor with an electron beam, has adopted a retarding method of applying a voltage to a workpiece for controlling the irradiation energy intensity of an electron beam. The technique has been described, for example, in Japanese Patent Laid-open Nos. Hei 05-258703 and Hei 06-188294.
However, a workpiece holder used for the electron beam application apparatus such as a length measuring SEM or analyzing SEM has failed to cope with a variation in electric field caused at an end portion of a workpiece due to a retarding voltage applied to the substrate. As a result, if it is intended to irradiate an end portion of a workpiece with an electron beam, the accuracy of the relationship between the irradiated position with the electron beam and the actual workpiece position is significantly degraded due to a variation in electric field, so that there occurs a problem that a portion near the end portion of the workpiece cannot be processed, analyzed or inspected.
A first object of the present invention is to solve the above problems of the prior art and to highly fast, stably acquire highly accurate images from irradiated positions with an electron beam on a circuit pattern at the step of fabricating a semiconductor device including an insulating material or a mixture of an insulating material and a conductive material, without occurrence of any deviation in the irradiated position in the images to be comparatively inspected, automatically comparing the images with each other thereby inspecting defects of the circuit pattern without occurrence of errors, and feeding back the result to the conditions of fabricating the semiconductor device thereby increasing the reliability of the semiconductor device and reducing the defective percentage thereof.
As a means for achieving the above object, a typical example of a circuit pattern inspecting apparatus of the present invention will be described.
The circuit pattern inspecting apparatus of the present invention basically includes an irradiation optical system for converging a primary charged particle beam while deflecting the primary charged particle beam so as to scan first and second regions of a circuit pattern of a workpiece with the primary charged particle beam; a retarding/accelerating means for retarding the primary charged particle beam, and accelerating secondary charged particles produced from the workpiece by the irradiation of the workpiece with the primary charged particle beam and reflection electrons reflected from the workpiece; a workpiece stage for holding the workpiece; a sensor for measuring the surface height of an irradiated position of the workpiece with the primary charged particle beam; a detector for detecting the secondary charged particles produced from the workpiece; and an image forming means for forming an image of the irradiated region of the workpiece from a signal detected by the detector. The apparatus further includes an outer peripheral sample having outer peripheral standard marks formed on at least two surfaces different in thickness in the direction of the axis of the primary charged particle beam, the outer peripheral sample being provided at an outer peripheral portion of a substrate mounting area of the workpiece stage; a central sample having central standard marks formed on at least two surfaces different in thickness in the direction of the axis of the primary charged particle beam, the central sample being provided at a central portion of the substrate mounting area of the workpiece stage; a storing means for storing image signals obtained from the outer peripheral standard marks of the outer peripheral sample and the central standard marks of the central sample; a computing means for computing a distortion amount of the primary charged particle beam inherent to the outer peripheral portion from the outer peripheral and central standard mark image signals; an eliminating means for eliminating the distortion amount inherent to the outer peripheral portion from the outer peripheral standard mark image signals; a storing means for preparing a deflection correcting table in accordance with the height of the workpiece on the basis of the outer peripheral standard mark image signals from, which the distortion amount has been eliminated, and storing the deflection correcting table; a deflection correcting signal generating circuit for taking out, a deflection correcting signal from the deflection correcting table in accordance with the surface height signal obtained by the sensor; and a control means for controlling to irradiate the outer peripheral sample having the outer peripheral standard marks at a desired timing to update the deflection correcting table.
In addition, preferably, a shield electrode is provided in the vicinity of the above substrate to reduce the disturbance of the electric field caused by a retarding voltage.
The function of the circuit pattern inspecting apparatus and circuit pattern inspecting method will be described below. The above circuit pattern inspecting apparatus compares the dependence of the workpiece height on the corrected amount of deflection at the central portion of the workpiece stage with that at the outer peripheral portion of a workpiece, to obtain a distortion amount inherent to the outer peripheral portion of the workpiece. Then, the dependence of height on the corrected amount of deflection at the outer peripheral portion of the workpiece is calculated by eliminating the distortion amount inherent to the outer peripheral portion from the standard mark signal of the outer peripheral portion, to obtain a deflection correcting amount at the outer peripheral portion which is equivalent to the deflection correcting amount obtained at the central portion.
Further, since a suitable deflection correcting table can be prepared only on the basis of the standard mark at the outer peripheral portion, the deflection correcting table can be updated by repeating desired times the calculation of the deflection correcting amount at the outer peripheral portion while holding the wafer on the workpiece stage. As a result, the deflection correcting table including the dependence of surface height, which is capable of keeping up with the drift of the electron beam, can be accurately obtained without reducing the throughput.
On the other hand, at the outermost peripheral portion of a wafer, there exists a region in which the deflection cannot be corrected on the basis of the same correcting table to produce the positional deviation, and if the result of image comparison at the region is outputted, there is a possibility that a large number of misdetections occur. For this reason, the present invention is configured such that the inspection is not performed in a region in which the deflection cannot be corrected on the basis of the same correcting table. This allows highly accurate inspection with no misdetection.
The provision, in the vicinity of the workpiece, of a shield electrode having the same potential as the retarding voltage of the workpiece has makes it possible to reduce the disturbance of an electric field near the workpiece, and hence to more extend a region on the wafer in which deflection can be corrected on the basis of the same correcting table.
According to the circuit pattern inspecting apparatus having the above functions, it is possible to highly fast, stably acquire highly accurate images from irradiated positions with an electron beam on a circuit pattern at the step of fabricating a semiconductor device including an insulating material or a mixture of an insulating material and a conductive material, without occurrence of any deviation in the irradiated position under a condition that a high retarding voltage is applied to a substrate, and automatically comparing the images with each other, thereby inspecting defects of the circuit pattern without occurrence of errors.
A second object of the present invention is to provide an electron beam application apparatus having a function of controlling an electron beam irradiation energy with a retarding voltage, which is capable of irradiating a position of a workpiece with an electron beam without reducing the accuracy of the irradiated position.
An electron beam application apparatus includes a vacuum chamber in which a workpiece such as a semiconductor device is irradiated with an electron beam, a loader for carrying the workpiece in the vacuum chamber, a movable stage for allowing the workpiece to be mounted thereon and adjusting an irradiated position with the electron beam, a workpiece holder disposed between the stage and the workpiece for holding the workpiece, a power source for applying a retarding voltage to the workpiece, a position measuring device for measuring the moved amount or the position of the stage, an electron source and a deflector for irradiating the workpiece with an electron beam for processing or observing the workpiece, and an information processing device for observing, analyzing, or inspecting the workpiece by making use of information obtained by detecting reflection electrons or secondary electrons produced from the workpiece. The height of a boundary portion between a portion, on the electron beam incident side, of the workpiece holder and the workpiece is set to be substantially equal to the height of the workpiece surface. With this configuration, since the electric field distribution of the workpiece surface is nearly uniform up to the end portion of the workpiece, a variation in electric field caused by the retarding voltage can be prevented. As a result, it is possible to irradiate the entire surface with an electron beam without reducing the positional accuracy.